Linux Fu: Easier File Watching

In an earlier installment of Linux Fu, I mentioned how you can use inotifywait to efficiently watch for file system changes. The comments had a lot of alternative ways to do the same job, which is great. But there was one very easy-to-use tool that didn’t show up, so I wanted to talk about it. That tool is entr. It isn’t as versatile, but it is easy to use and covers a lot of common use cases where you want some action to occur when a file changes.

The program is dead simple. It reads a list of file names on its standard input. It will then run a command and repeat it any time the input files change. There are a handful of options we’ll talk about in a bit, but it is really that simple. For example, try this after you install entr with your package manager.

Open two shell windows

In one window, open your favorite editor to create an empty file named /tmp/foo and save it

In the second window issue the command: echo "/tmp/foo" | entr wc /tmp/foo

Back in the first window (or your GUI editor) make some changes to the file and save it while observing the second window